Electrostatic discharge protection of thin-film resonators

ABSTRACT

A filter having a thin-film resonator fabricated on a semiconductor substrate and a method of making the same are disclosed. The filter has a bonding pad connected to the resonator and in contact with the substrate to form a Schottky diode with the substrate to protect the resonator from electrostatic discharges.

BACKGROUND

[0001] The present invention relates to acoustic resonators, and moreparticularly, to resonators that may be used as filters for electroniccircuits.

[0002] The need to reduce the cost and size of electronic equipment hasled to a continuing need for ever-smaller electronic filter elements.Consumer electronics such as cellular telephones and miniature radiosplace severe limitations on both the size and cost of the componentscontained therein. Further, many such devices utilize electronic filtersthat must be tuned to precise frequencies. Filters select thosefrequency components of electrical signals that lie within a desiredfrequency range to pass while eliminating or attenuating those frequencycomponents that lie outside the desired frequency range.

[0003] One class of electronic filters that has the potential formeeting these needs is constructed from thin film bulk acousticresonators (FBARs). These devices use bulk longitudinal acoustic wavesin thin film piezoelectric (PZ) material. In one simple configuration, alayer of PZ material is sandwiched between two metal electrodes. Thesandwich structure is preferably suspended in air. A sampleconfiguration of an apparatus 10 having a resonator 12 (for example, anFBAR) is illustrated in FIGS. 1A and 1B. FIG. 1A illustrates a top viewof the apparatus 10 while FIG. 1B illustrates a side view of theapparatus 10 along line A-A of FIG. 1A. The resonator 12 is fabricatedabove a substrate 14. Deposited and etched on the substrate 14 are, inorder, a bottom electrode layer 15, piezoelectric layer 17, and a topelectrode layer 19. Portions (as indicated by brackets 12) of theselayers—15, 17, and 19—that overlap and are fabricated over a cavity 22constitute the resonator 12. These portions are referred to as a bottomelectrode 16, piezoelectric portion 18, and a top electrode 20. In theresonator 12, the bottom electrode 16 and the top electrode 20sandwiches the PZ portion 18. The electrodes 14 and 20 are conductorswhile the PZ portion 18 is typically crystal such as Aluminum Nitride(AlN).

[0004] When electric field is applied between the metal electrodes 16and 20, the PZ portion 18 converts some of the electrical energy intomechanical energy in the form of mechanical waves. The mechanical wavespropagate in the same direction as the electric field and reflect off ofthe electrode/air interface.

[0005] At a resonant frequency, the resonator 12 acts as an electronicresonator. The resonant frequency is the frequency for which the halfwavelength of the mechanical waves propagating in the device isdetermined by many factors including the total thickness of theresonator 12 for a given phase velocity of the mechanical wave in thematerial. Since the velocity of the mechanical wave is four orders ofmagnitude smaller than the velocity of light, the resulting resonatorcan be quite compact. Resonators for applications in the GHz range maybe constructed with physical dimensions on the order of less than 100microns in lateral extent and a few microns in total thickness. Inimplementation, for example, the resonator 12 is fabricated using knownsemiconductor fabrication processes and is combined with electroniccomponents and other resonators to form electronic filters forelectrical signals.

[0006] The use and the fabrication technologies for various designs ofFBARs for electronic filters are known in the art and a number ofpatents have been granted. For example, U.S. Pat. No. 6,262,637 grantedto Paul D. Bradley et al. discloses a duplexer incorporating thin-filmbulk acoustic resonators (FBARs). Various methods for fabricating FBARsalso have been patented, for example, U.S. Pat. No. 6,060,181 granted toRichard C. Ruby et al. discloses various structures and methods offabricating resonators, and U.S. Pat. No. 6,239,536 granted to KennethM. Lakin discloses method for fabricating enclosed thin-film resonators.

[0007] However, the continuing drive to increase the quality andreliability of the FBARs presents challenges requiring even betterresonator quality, designs, and methods of fabrication. For example, onesuch challenge is to eliminate or alleviate susceptibility of the FBARsfrom damages from electrostatic discharges and voltage spikes fromsurrounding circuits. Another challenge is to eliminate or alleviatesusceptibility of the resonator from frequency drifts due to interactionwith its environment such as air or moisture.

SUMMARY

[0008] These and other technological challenges are met by the presentinvention. According to one aspect of the present invention, anelectronic filter includes a thin-film resonator fabricated on asemiconductor substrate and bonding pad connected to the thin-filmresonator, the bonding pad forming a Schottky diode with the substrateto protect the thin-film resonator from electrostatic discharges.

[0009] According to another aspect of the present invention, a methodfor fabricating an electronic filter is disclosed. A thin-film resonatoris fabricated on a substrate. With the resonator, a bonding pad isfabricated, the bonding pad being connected to the thin-film resonatorand a portion of the bonding-pad being in contact with the substrate toform a Schottky diode.

[0010] Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1A is a top view of an apparatus including a resonator knownin prior art;

[0012]FIG. 1B is a side view of the apparatus of FIG. 1A cut along lineA-A;

[0013]FIG. 2A is a top view of an apparatus according to a firstembodiment of the present invention;

[0014]FIG. 2B is a side view of the apparatus of FIG. 2A cut along lineB-B;

[0015]FIG. 3A is a top view of an apparatus according to a secondembodiment of the present invention;

[0016]FIG. 3B is a side view of the apparatus of FIG. 3A cut along lineC-C;

[0017]FIG. 4A is a top view of an apparatus according to a thirdembodiment of the present invention;

[0018]FIG. 4B is a side view of the apparatus of FIG. 4A cut along lineD-D; and

[0019]FIG. 4C is a schematic diagram illustrating, in part, a circuitthat can be formed using the apparatus of FIG. 4A.

DETAILED DESCRIPTION

[0020] As shown in the drawings for purposes of illustration, thepresent invention is embodied in a filter having a thin-film resonatorfabricated on a semiconductor substrate. Bonding pad connecting theresonator to the rest of the filter circuit is connected to theresonator. The bonding pad is in contact with the substrate therebyforming a Schottky junction diode. In normal operations, the diode is anopen circuit and does not affect the operations of the filter. When anelectrostatic discharge (ESD) spike voltage is introduced to theresonator via the bonding pad, the diode closes thereby discharging theESD voltage to the substrate thereby protecting the resonator.

[0021]FIG. 2A illustrates a top view of an apparatus 30 according to afirst embodiment of the present invention. FIG. 2B is a side view of theapparatus 30 of FIG. 2A cut along line B-B. Portions of the apparatus 30in FIGS. 2A and 2B are similar to those of the apparatus 10 of FIGS. 1Aand 1B. For convenience, portions of the apparatus 30 in FIGS. 2A and 2Bthat are similar to portions of the apparatus 10 of FIGS. 1A and 1B areassigned the same reference numerals and different portions are assigneddifferent reference numerals. Referring to FIGS. 2A and 2B, theapparatus 30 according to one embodiment of the present inventionincludes a resonator 32 fabricated on a substrate 14. The apparatus 30is fabricated first be etching a cavity 34 into the substrate 14 andfilling it with suitable sacrificial material such as, for example,phosphosilicate glass (PSG). Then, the substrate 14, now including thefilled cavity 34 is planarized using known methods such as chemicalmechanical polishing. The cavity 34 can include an evacuation tunnelportion 34 a aligned with an evacuation via 35 through which thesacrificial material is later evacuated.

[0022] Next, a thin seed layer 38 is fabricated on the planarizedsubstrate 14. Typically the seed layer 38 is sputtered on the planarizedsubstrate 14. The seed layer 38 can be fabricated using Aluminum Nitride(AlN) or other similar crystalline material, for example, AluminumOxynitride (ALON), Silicon Dioxide (SiO₂), Silicon Nitride (Si₃N₄), orSilicon Carbide (SiC). In the illustrated embodiment, the seed layer 38is in the range of about 10 Angstroms (or one nanometer) to 10,000Angstroms (or one micron) thick. Techniques and the processes offabricating a seed layer are known in the art. For example, the widelyknown and used sputtering technique can be used for this purpose.

[0023] Then, above the seed layer 38, the following layers aredeposited, in order: a bottom electrode layer 15, a piezoelectric layer17, and a top electrode layer 19. Portions (as indicated by brackets 32)of these layers—36, 15, 17, and 19—that overlap and are situated abovethe cavity 34 constitute the resonator 32. These portions are referredto as a seed layer portion 40, bottom electrode 16, piezoelectricportion 18, and top electrode 20. The bottom electrode 16 and the topelectrode 20 sandwiches the PZ portion 18.

[0024] The electrodes 14 and 20 are conductors such as Molybdenum and,in a sample embodiment, are in a range of 0.3 micron to 0.5 micronthick. The PZ portion 18 is typically made from crystal such as AluminumNitride (AlN), and, in the sample embodiment, is in a range from 0.5micron to 1.0 micron thick. From the top view of the resonator 32 inFIG. 2A, the resonator can be about 150 microns wide by 100 micronslong. Of course, these measurements can vary widely depending on anumber of factors such as, without limitation, the desired resonantfrequency, materials used, the fabrication process used, etc. Theillustrated resonator 32 having these measurements can be useful infilters in the neighborhood of 1.92 GHz. Of course, the presentinvention is not limited to these sizes or frequency ranges.

[0025] The fabrication of the seed layer 38 provides for a betterunderlayer on which the PZ layer 17 can be fabricated. Accordingly, withthe seed layer 38, a higher quality PZ layer 17 can be fabricated, thusleading to a higher quality resonator 32. In fact, in the present sampleembodiment, the material used for the seed layer 38 and the PZ layer 17are the same material, AlN. This is because seed layer 38 nucleates asmoother, more uniform bottom electrode layer 15 which, in turn,promotes a more nearly single crystal quality material for the PZ layer17. Thus, piezoelectric coupling constant of the PZ layer 17 isimproved. The improved piezoelectric coupling constant allows for widerbandwidth electrical filters to be built with the resonator 32 and alsoyields more reproducible results since it tightly approaches thetheoretical maximum value for AlN material.

[0026]FIG. 3A illustrates a top view of an apparatus 50 according to asecond embodiment of the present invention. FIG. 3B is a side view ofthe apparatus 50 of FIG. 3A cut along line C-C. Portions of theapparatus 50 in FIGS. 3A and 3B are similar to those of the apparatus 30of FIGS. 2A and 2B. For convenience, portions of the apparatus 50 inFIGS. 3A and 3B that are similar to portions of the apparatus 30 ofFIGS. 2A and 2B are assigned the same reference numerals and differentportions are assigned different reference numerals.

[0027] Referring to FIGS. 3A and 3B, the apparatus 50 of the presentinvention includes a resonator 52 fabricated on a substrate 14. Theapparatus 50 is fabricated similarly to the apparatus 30 of FIGS. 2A and2B and discussed herein above. That is, bottom electrode layer 15,piezoelectric layer 17, and top electrode layer 19 are fabricated abovea substrate 14 having a cavity 34. Optionally, a seed layer 38 isfabricated between the substrate 14 including the cavity 34 and thebottom electrode layer 15. Details of these layers are discussed above.The resonator 52 comprises portions (as indicated by brackets 52) ofthese layers—36, 15, 17, and 19—that overlap and are situated above thecavity 34. These portions are referred to as a seed layer portion 40,bottom electrode 16, piezoelectric portion 18, and top electrode 20.Finally, a protective layer 54 is fabricated immediately above the topelectrode 20. The protective layer (54 covers, at least, the topelectrode 20, and can cover, as illustrated, a larger area than the topelectrode 20. Moreover, portion of the protective layer 54 that issituated above the cavity 34 is also a part of the resonator 52. Thatis, that portion of the protective layer 54 contributes mass to theresonator 52 and resonates with all the other parts—40, 16, 18, and20—of the resonator 52.

[0028] The protective layer 54 chemically stabilizes and reduces thetendency of material to adsorb on the surface of the top electrode 20.Adsorbed material can change the resonant frequency of the resonator 32.The thickness may also be adjusted to optimize the electrical qualityfactor (q) of the resonator 32.

[0029] Without the protective layer 54, resonant frequency of theresonator 52 is relatively more susceptible to drifting over time. Thisis because the top electrode 20, a conductive metal, can oxidize fromexposure to air and potentially moisture. The oxidization of the topelectrode 20 changes the mass of the top electrode 20 thereby changingthe resonant frequency. To reduce or minimize the resonantfrequency-drifting problem, the protective layer 54 is typicallyfabricated using inert material less prone to reaction with theenvironment such as Aluminum Oxynitride (ALON), Silicon Dioxide (SiO₂),Silicon Nitride (Si₃N₄), or Silicon Carbide (SiC). In experiments, theprotective layer 54 having thickness ranging from 30 Angstroms to to 2microns have been fabricated. The protective layer 54 can include AlNmaterial, which can also be used for the piezoelectric layer 17.

[0030] Here, the seed layer portion 40 not only improves the crystallinequality of the resonator 52, but also serves as a protective underlayerprotecting the bottom electrode 16 from reaction with air and possiblemoisture from the environment reaching the bottom electrode 16 via theevacuation via 35.

[0031]FIG. 4A illustrates a top view of an apparatus 60 according to athird embodiment of the present invention. FIG. 4B is a side view of theapparatus 60 of FIG. 4A cut along line D-D. FIG. 4C is a simpleschematic illustrating, in part, an equivalent circuit that can beformed using the apparatus 60. Portions of the apparatus 60 in FIGS. 4A,4B, and 4C are similar to those of the apparatus 10 of FIGS. 1A and 1Band the apparatus 30 of FIGS. 2A and 2B. For convenience, portions ofthe apparatus 60 in FIGS. 4A, 4B, and 4C that are similar to portions ofthe apparatus 10 of FIGS. 1A and 1B and portions of the apparatus 30 ofFIGS. 2A and 2B are assigned the same reference numerals and differentportions are assigned different reference numerals.

[0032] Referring to FIGS. 4A, 4B, and 4C, the apparatus 60 is fabricatedsimilarly to the apparatus 10 of FIGS. 1A and 1B and discussed hereinabove. That is, bottom electrode layer 15, piezoelectric layer 17, andtop electrode layer 19 are fabricated above a substrate 14 having acavity 22. These layers are fabricated in a similar manner as theapparatus 30 of FIGS. 2A and 2B and the details of these layers arediscussed above. The resonator 12, preferably a thin-film resonator suchas an FBAR, comprises portions (as indicated by brackets 12) of theselayers—15, 17, and 19—that overlap and are situated above the cavity 22.These portions are referred to as bottom electrode 16, piezoelectricportion 18, and top electrode 20.

[0033] The apparatus 60 includes at least one bonding pad. Illustratedin FIGS. 4A and 4B are a first bonding pad 62 and a second bonding pad64. The first bonding pad 62 is connected to the resonator 12 by its topelectrode layer 19. The first boding pad 62 is in contact with thesemiconductor substrate 14 thereby forming a Schottky junction diode 63.Operational characteristics of such diodes are known in the art.

[0034] Also illustrated is a second bonding pad 64 connected to theresonator 12 by its bottom electrode layer 15. The second bonding pad 64is illustrated as making contact with the substrate 14 at two placesthereby forming two Schottky diode contacts 65. In fact, a bonding padcan be fabricated to form, in combination with the substrate 14, aplurality of diode contacts for the protection of the resonator to whichit is connected. The contacts 65 from a single pad 64 form,electrically, a single Schottky diode.

[0035] The bonding pads 62, 64 are typically fabricated using conductivemetal such as gold, nickel, chrome, other suitable materials, or anycombination of these.

[0036]FIG. 4C can be used to used to describe the operations of thefilter circuit 72 having the resonator 12. Normally, no current flowsthrough the diodes 63 and 65 as the diode 63 operate as an open circuitin one direction while diode 65 operates as a closed circuit in theopposite direction. However, when an electrostatic voltage spike isintroduced to the resonator 12 via its bonding pad 64 (from, perhaps, anantennae 66), the diode 63 breaks down. When the diode 63 breaks down,it is effectively a closed short circuit, and allows the voltage spiketo be transferred to the substrate 14, and eventually ground 68, therebyprotecting the resonator 12 from the voltage spike. The other diode 65operates similarly to protect the resonator 12 from voltage spikes fromother electronic circuits 70 connected to the filter 72. That is, twometal pads, for example pads 62 and 64 connected to electricallyopposing sides of the resonator 12, fabricated on semiconductorsubstrate create an electrical circuit of two back-to-back Schottkydiodes which allow high voltage electrostatic discharges to dissipateharmlessly in the substrate rather than irreversibly breaking down thepiezoelectric layer, for example PZ layer 17, which separates top andbottom electrodes, for example electrodes 16 and 20, from each other. Anelectronic schematic diagram of FIG. 4C illustrates such connection.

[0037] In an alternative embodiment, a single apparatus can include aresonator having all of the features discussed above including the seedlayer 38 and the protective layer 54 illustrated in FIGS. 2A, 2B, 3A and3B and bonding pads 62 and 64 (forming Shottkey diodes 63 and 65)illustrated in FIGS. 4A and 4B. In the alternative embodiment, the pads62 and 64 can be formed on the seed layer 38 with several microns ofoverhang over and beyond the top electrode layer 19 and the bottomelectrode layer 15.

[0038] From the foregoing, it will be appreciated that the presentinvention is novel and offers advantages over the current art. Althougha specific embodiment of the invention is described and illustratedabove, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The invention islimited by the claims that follow.

What is claimed is:
 1. An apparatus comprising: a thin-film resonatorfabricated on a semiconductor substrate; bonding pad connected to saidthin-film resonator, the bonding pad forming a Schottky diode with thesubstrate to protect said thin-film resonator from electrostaticdischarges.
 2. The apparatus recited in claim 1 wherein said boding padforms a plurality of Schottky diodes with the substrate.
 3. Theapparatus recited in claim 1 wherein said boding pad comprises aconductive material.
 4. The apparatus recited in claim 1 wherein saidboding pad comprises conductor selected from a group consisting of gold,nickel, and chrome.
 5. The apparatus recited in claim 1 wherein saidthin-film resonator comprises piezoelectric portion sandwiched by abottom electrode and a top electrode.
 6. The apparatus recited in claim5 wherein the piezoelectric portion comprises Aluminum Nitride and saidbottom and top electrodes comprises Molybdenum.
 7. A method forfabricating an apparatus, the method comprising: fabricating a thin-filmresonator on a substrate; fabricating a bonding pad connected to saidthin-film resonator, a portion of said bonding pad in contact with thesubstrate to form a Schottky diode.
 8. The method recited in claim 7wherein said boding pad forms a plurality of Schottky diodes with thesubstrate.
 9. The method recited in claim 7 wherein said boding padcomprises a conductive material.
 10. The method recited in claim 7wherein said boding pad comprises conductor selected from a groupconsisting of gold, nickel, and chrome.
 11. The method recited in claim7 wherein said thin-film resonator comprises piezoelectric portionsandwiched by a bottom electrode and a top electrode.
 12. The methodrecited in claim 11 wherein the piezoelectric portion comprises AluminumNitride and said bottom and top electrodes comprises Molybdenum.